The base algorithm works as follows: SQL Server compares the first rows from both sorted inputs. It then continues comparing the next rows from the second input as long as the values match the first input’s value.
Once the values no longer match, SQL Server increments the row of whichever input has the smaller value – it then continues performing comparisons and outputting any joined records. (For more detailed information, be sure to check out Craig Freedman’s post on merge joins.)
Beware: Bert has animations which might cause you to space out for a few hours.